1. Field of the Invention
The present invention relates to a fuel cell stack.
2. Description of the Related Art
A solid polymer electrolyte fuel cell, for example, includes unit cells. Each of the unit cells includes a membrane electrode assembly (MEA) and separators sandwiching the MEA therebetween. The MEA includes an electrolyte membrane (electrolyte), which is a polymer ion-exchange membrane, and an anode electrode and a cathode electrode sandwiching the electrolyte membrane therebetween.
When using this type of fuel cell for an automobile, a certain number (for example, tens or hundreds) of unit cells are stacked so as to form a fuel cell stack that generates a required electric power. In general, such a fuel cell stack has a reactant gas channel and a reactant gas manifold. A reactant gas flows through the reactant gas channel along a surface of a separator and along a power generation surface. The reactant gas manifold is connected to the reactant gas channel and extends in the direction in which the unit cells are stacked. This is so-called an internal manifold structure.
Such an internal manifold type fuel cell stack has a problem in that water, which is generated by a power generating reaction, may accumulate, in particular, near an outlet manifold (reactant gas outlet manifold). Japanese Patent No. 4062797, for example, discloses a solid polymer electrolyte fuel cell having a structure for avoiding instable operation due to flooding.
Japanese Patent No. 4062797 describes a solid polymer electrolyte fuel cell including a gas manifold integrated separator. The gas manifold integrated separator includes a gas inlet manifold hole, a gas outlet manifold hole, a gas channel groove section, a gas inlet that connects the gas inlet manifold hole to the gas channel groove section, and a gas outlet that connects the gas outlet manifold hole to the gas channel groove section.
This fuel cell has a feature of having the manifold integrated separators, which sandwich the membrane electrode assembly, wherein the width of the gas channel groove section of at least the gas inlet and the gas outlet adjacent to the gas manifold is increased.
It is described that, with such a structure, the fluid resistance in the connection portion between the gas channel groove section and the gas manifold hole is decreased, and even if a part of water in the gas is condensed in the gas channel groove section, the condensed water can be easily discharged through the gas outlet to the gas manifold hole.
However, in the fuel cell described in Japanese Patent No. 4062797, because the width of the gas outlet adjacent to the gas outlet manifold hole is increased, condensed water is discharged in a continuous state through the gas outlet to the gas manifold hole. Therefore, a liquid junction occurs between unit cells that are stacked. In particular, if the separators are made of a metal, corrosion currents are easily generated on surfaces of the metal separators due to a potential difference.
As a result, the metal separators corrode and metal ions are emitted, which leads to a decrease in the performance of the electrolyte membrane. Moreover, when the metal separator becomes thinner due to the corrosion, a hole may form in the metal separator, which leads to a decrease in gas shielding capability.